What construction material did the Romans use most often in their engineering projects?

concrete

concrete comprised of mortar and aggregate

Explanation:

The Romans are known for their remarkable engineering feats, be they roads, bridges, tunnels, or their impressive aqueducts. Their constructions, many of them still standing, are a testament to their superior engineering skills and ingenuity.

Roman construction materials. Roman construction is famed for the use of concrete and the buildings at Portus are no exception. Roman concrete is composed of mortar and aggregate. The mortar was a mixture of lime and a volcanic sand called pozzolana.